Shut Down Valve Timingis 24" in 24s OK?

[boboG]

We are converting a 24" HV (rotary)to an air actuated shut down valve. Is it Ok to apply same rule of thumb (approx 1s per inch) for closure time as control valves to a shut down valve? Our process guys seem to be OK with this but I think we should specify a faster closure time, any guidelines or recom. practice I can refer to? The valve is pneumatic spring return.

Thanks for your help

 

[BigInch]

Process guys usually don't mind faster valve closing times, until they are underneath a connecting pipe that bangs away when somebody closes it. Hydraulics guys don't like valves moving fast in any direction.

Closing time of 1 sec/inch_diameter is about as fast as you would want a valve to close, unless it is in snap acting ESD service. I usually start a transient analysis using that 1 s/in_D as a default speed value, to see if there are any transient problems when I open or close it during a pipeline simulation. If there are no problems, I will leave it at that speed in my simulation, because normally, they will not be set to operate any faster than that and my results will be conservative.

The speed of a valve should not be increased to the point where transient pressures become significant. High transient pressure waves can be generated when valve closure times are too fast.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[terje61]

BigInch is correct. Do not close faster than necessary. 1 inch per sec. is a good rule of thumb.

However for safety or protection reasons fast stroking times are often required. For gas applications the EN 14382 (formerly DIN 3380) specifies < 2sec.

Terje

 

[BigInch]

terj, thanks for adding the DIN, EN spec.

And if I can add....

[COLOR=white red]It can be very important to consider the valve characteristics. [/color] A typical ball valve, for example, will travel 95% (or even more) of its total distance without any appreciable shutoff. Then ALL flow changes occur in the last couple of seconds traveling the ultimate 5% of travel. That's where shut-down timing becomes very important, because in actuality the valve is closing in not 24 seconds, BUT [COLOR=white red]1.2 SECONDS![/color]


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[JLSeagull]

Without checking the specific phrase, the offshore requirement is for shutdown valves to close within 45 seconds. I inspected a large group of shutdown valves and found one second per inch NPS to be about right. These valves excluded the pneumatic speed controls. The actuator packager needs to do things to quick exhaust etc. as you move to larger valves, perhaps 30 NPS and larger. BTW, they take longer to close against 1000 psig process gas than atmospheric air in the shop. I bet that oil or other viscous liquids are still slower.

 

[boboG]

Thank you guys for your assurance, I guess we'll stick to that rule of thumb then. BTW the service is H2S gas service and I will classify the valve as doing a shutoff/plant segregation function hence the safety case might apply. we have recently discovered that timing problem is the least of our worries in retrofitting an actuator the valve it seems the actiator is too strong for the valve!!

 

[JimCasey]

Good point from Big Inch: even full-port ball valves have something very close to equal-percentage characteristic, and meaningful restriction only happens at small openings.

LIQUID service valves need to shut slowly to avoid water hammer. GAS SERVICE valves can shut quickly. You do not need to artificially slow an actuator for a normal on-off service gas valve.

Here in the secret valve factory we sometimes supply 36" butterfly valves for ESD and we have gotten down to 0.2 seconds when required. (LARGE Pneumatic actuator, GYRONOMOUS spring-to-close, REALLY big air ports) Admittedly you don't want to do too many cycles on a valve closing that quickly and the local seismographic office finds it considerate if we tell them a test like that is coming. Our asembly workers in adjacent cells in the plant like a little warning, too.

It >is< possible to snap a big valve shut quickly if necessary on Gas.

 

[BigInch]

I use snap action valves on liquid service, but I [red]DO the transient analysis [/color] before writing the actuator spec. They are usually N2 actuated ESD stations.

H2S, go for a fast closure in ESD applications! Gas is usually not a problem, unless you have a tremendous flow. 24" might want to check, but you can handle a lot more fast timers than a liquid line. Usually gas transients are more important in their affect on equipment around them, compressor surging, etc. rather than line stress.



Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

If you need to close a valve quickly, you do. If there is a need, there is a need. A "water hammer" is not good, but if the alternative is worse, then you do it.

If not, then 1 sec/inch is good.

Most of the valves that I am recently working on are 16" and under, and most close in about 1 minute.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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